Unfortunately, that's not how the logic works.
Instead, it behaves like this:
Did Q hit *an**any* enemy?
-- Yes -> Reduce Cooldown by 1
-- No -> break;
For this same reason, a Nasus who kills multiple minions with his Q (due to Tiamat or Frostborn Gauntlet (or both!) ) will only power up Siphoning Strike by one minion's worth of damage.